The nuclear enzyme poly(ADP-ribose) polymerase-1 (PARP-1) is a member of the PARP enzyme family. This growing family of enzymes consist of PARPs such as, for example: PARP-1, PARP-2, PARP-3 and Vault-PARP; and Tankyrases (TANKs), such as, for example: TANK-1 and TANK-2. PARP is also referred to as poly(adenosine 5′-diphospho-ribose) polymerase or PARS (poly(ADP-ribose) synthetase).
PARP-1 is a major nuclear protein of 116 kDa consisting of three domains: an N-terminal DNA binding domain containing two zinc fingers, an automodification domain and a C-terminal catalytic domain. The enzyme synthesizes poly(ADP-ribose), a branched polymer that can consist of over 200 ADP-ribose units. The protein acceptors of poly(ADP-ribose) are directly or indirectly involved in maintaining DNA integrity. They include histones, HMG proteins, topoisomerases, DNA and RNA polymerases, DNA ligases, Ca2+- and Mg2+-dependent endonucleases and single-strand break-repair and base-excision repair factors. PARP protein is expressed at a high level in many tissues, most notably in the immune system, heart, brain and germ-line cells. Under normal physiological conditions, there is minimal PARP activity. However, DNA damage causes an immediate activation of PARP by up to 500-fold. The resulting poly(ADP-ribose) production has three consequences: first, DNA-damage-induced poly(ADP-ribosyl)ation of the N- and C-terminal tails of histone H1 and H2B or the selective interaction of these proteins with free or PARP-1 bound poly(ADP-ribose) contributes to the relaxation of the 30-nm chromatin fibre and increases the access to breaks; second, it signals the occurrence and the extent of DNA damage so that the cell can establish an adaptive response according to the severity of the injury (DNA repair or cell suicide); third, it mediates the fast recruitment of single-strand break-repair and base-excision repair factors.
Single strand breaks (SSBs) occur spontaneously in all cells. In the absence of PARP-1 activity these SSBs may be converted to double strand breaks (DSBs) during replication that can lead to collapse of the replication forks. DSBs are identified by their epigenetic mark, the phosphorylation of the core histone variant H2AX (γH2AX). The very rapid local decondensation of chromatin, which occurs in a γ H2AX-independent manner at DSB's can be attributed to poly(ADP-ribose) production that is mediated locally by PARP-1.
Also developmental or environmental cues, such as steroids or heat shock, induce PARP-1 activation and the poly(ADP-ribose)-dependent stripping of histones from chromatin, thereby favouring the opening of the chromatin structure, which may allow transcriptional activation in the absence of DNA breaks.
Extensive PARP activation in cells suffering from massive DNA damage leads to severe depletion of NAD+. The short half-life of poly(ADP-ribose) results in a rapid turnover rate. Once poly(ADP-ribose) is formed, it is quickly degraded by the constitutively active poly(ADP-ribose) glycohydrolase (PARG), together with phosphodiesterase and (ADP-ribose) protein lyase. PARP and PARG form a cycle that converts a large amount of NAD+ to ADP-ribose. In less than an hour, over-stimulation of PARP can cause a drop of NAD+ and ATP to less than 20% of the normal level. Such a scenario is especially detrimental during ischaemia when deprivation of oxygen has already drastically compromised cellular energy output. Subsequent free radical production during reperfusion is assumed to be a major cause of tissue damage. Part of the ATP drop, which is typical in many organs during ischaemia and reperfusion, could be linked to NAD+ depletion due to poly(ADP-ribose) turnover. Thus, PARP or PARG inhibition is expected to preserve the cellular energy level thereby potentiating the survival of ischaemic tissues after insult.
Poly(ADP-ribose) synthesis is also involved in the induced expression of a number of genes essential for inflammatory response. PARP inhibitors suppress production of inducible nitric oxide synthase (iNOS) in macrophages, P-type selectin and intercellular adhesion molecule-1 (ICAM-1) in endothelial cells. Such activity underlies the strong anti-inflammation effects exhibited by PARP inhibitors. PARP inhibition is able to reduce necrosis by preventing translocation and infiltration of neutrophils to the injured tissues.
PARP is activated by damaged DNA fragments and, once activated, catalyzes the attachment of up to 100 ADP-ribose units to a variety of nuclear proteins, including histones and PARP itself. During major cellular stresses the extensive activation of PARP can rapidly lead to cell damage or death through depletion of energy stores. As four molecules of ATP are consumed for every molecule of NAD+ regenerated, NAD+ is depleted by massive PARP activation, in the efforts to re-synthesize NAD+, ATP may also become depleted.
It has been reported that PARP activation plays a key role in both NMDA- and NO-induced neurotoxicity. This has been demonstrated in cortical cultures and in hippocampal slices wherein prevention of toxicity is directly correlated to PARP inhibition potency. The potential role of PARP inhibitors in treating neurodegenerative diseases and head trauma has thus been recognized even if the exact mechanism of action has not yet been elucidated.
Similarly, it has been demonstrated that single injections of PARP inhibitors have reduced the infarct size caused by ischemia and reperfusion of the heart or skeletal muscle in rabbits. In these studies, a single injection of 3-amino-benzamide (10 mg/kg), either one minute before occlusion or one minute before reperfusion, caused similar reductions in infarct size in the heart (32-42%) while 1,5-dihydroxyisoquinoline (1 mg/kg), another PARP inhibitor, reduced infarct size by a comparable degree (38-48%) These results make it reasonable to assume that PARP inhibitors could salvage previously ischaemic heart or reperfusion injury of skeletal muscle tissue.
PARP activation can also be used as a measure of damage following neurotoxic insults resulting from exposure to any of the following inducers like glutamate (via NMDA receptor stimulation), reactive oxygen intermediates, amyloid β-protein, N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) or its active metabolite N-methyl-4 phenylpyridine (MPP+), which participate in pathological conditions such as stroke, Alzheimer's disease and Parkinson's disease. Other studies have continued to explore the role of PARP activation in cerebellar granule cells in vitro and in MPTP neurotoxicity. Excessive neural exposure to glutamate, which serves as the predominate central nervous system neurotransmitter and acts upon the N-methyl D-aspartate (NMDA) receptors and other subtype receptors, most often occurs as a result of stroke or other neurodegenerative processes. Oxygen deprived neurons release glutamate in great quantities during ischaemic brain insult such as during a stroke or heart attack. This excess release of glutamate in turn causes over-stimulation (excitotoxicity) of N-methyl-D-aspartate (NMDA), AMPA, Kainate and MGR receptors, which open ion channels and permit uncontrolled ion flow (e.g., Ca2+ and Na+ into the cells and K+ out of the cells) leading to overstimulation of the neurons. The over-stimulated neurons secrete more glutamate, creating a feedback loop or domino effect which ultimately results in cell damage or death via the production of proteases, lipases and free radicals. Excessive activation of glutamate receptors has been implicated in various neurological diseases and conditions including epilepsy, stroke, Alzheimer's disease, Parkinson's disease, Amyotrophic Lateral Sclerosis (ALS), Huntington's disease, schizophrenia, chronic pain, ischemia and neuronal loss following hypoxia, hypoglycemia, ischemia, trauma, and nervous insult. Glutamate exposure and stimulation has also been implicated as a basis for compulsive disorders, particularly drug dependence. Evidence includes findings in many animal species, as well as in cerebral cortical cultures treated with glutamate or NMDA, that glutamate receptor antagonists (i.e., compounds which block glutamate from binding to or activating its receptor) block neural damage following vascular stroke. Attempts to prevent excitotoxicity by blocking NMDA, AMPA, Kainate and MGR receptors have proven difficult because each receptor has multiple sites to which glutamate may bind and hence finding an effective mix of antagonists or universal antagonist to prevent binding of glutamate to all of the receptor and allow testing of this theory, has been difficult. Moreover, many of the compositions that are effective in blocking the receptors are also toxic to animals. As such, there is presently no known effective treatment for glutamate abnormalities.
The stimulation of NMDA receptors by glutamate, for example, activates the enzyme neuronal nitric oxide synthase (nNOS), leading to the formation of nitric oxide (NO), which also mediates neurotoxicity. NMDA neurotoxicity may be prevented by treatment with nitric oxide synthase (NOS) inhibitors or through targeted genetic disruption of nNOS in vitro.
Another use for PARP inhibitors is the treatment of peripheral nerve injuries, and the resultant pathological pain syndrome known as neuropathic pain, such as that induced by chronic constriction injury (CCI) of the common sciatic nerve and in which transsynaptic alteration of spinal cord dorsal horn characterized by hyperchromatosis of cytoplasm and nucleoplasm (so-called “dark” neurons) occurs.
Evidence also exists that PARP inhibitors are useful for treating inflammatory bowel disorders, such as colitis. Specifically, colitis was induced in rats by intraluminal administration of the hapten trinitrobenzene sulfonic acid in 50% ethanol. Treated rats received 3-aminobenzamide, a specific inhibitor of PARP activity. Inhibition of PARP activity reduced the inflammatory response and restored the morphology and the energetic status of the distal colon.
Further evidence suggests that PARP inhibitors are useful for treating arthritis. Further, PARP inhibitors appear to be useful for treating diabetes. PARP inhibitors have been shown to be useful for treating endotoxic shock or septic shock.
PARP inhibitors have also been used to extend the lifespan and proliferative capacity of cells including treatment of diseases such as skin aging, Alzheimer's disease, atherosclerosis, osteoarthritis, osteoporosis, muscular dystrophy, degenerative diseases of skeletal muscle involving replicative senescence, age-related muscular degeneration, immune senescence, AIDS, and other immune senescence disease; and to alter gene expression of senescent cells.
Tankyrases (TANKs) were identified as components of the human telomeric complex. They have also been proposed to have roles in regulation of the mitotic spindle and in vesicle trafficking and they may serve as scaffolds for proteins involved in various other cellular processes. Telomeres, which are essential for chromosome maintenance and stability, are maintained by telomerase, a specialized reverse transcriptase. TANKs are (ADP-ribose)transferases with some features of both signalling and cytoskeletal proteins. They contain the PARP domain, which catalyses poly-ADP-ribosylation of substrate proteins, the sterile alpha motif, which is shared with certain signalling molecules and the ANK domain, which contains 16 to 24 ankyrin repeats, also present in the cytoskeletal protein ankyrin. The ANK domain interacts with a variety of different proteins, including the telomeric protein, Telomere Repeat binding Factor-1 (TRF-1). These proteins were therefore named TRF1-interacting, ankyrin-related ADP-ribose polymerases (TANKs).
One function of TANKs is the ADP-ribosylation of TRF-1. Human telomere function is regulated by a complex of telomere associated proteins that includes the two telomere-specific DNA binding proteins, TRF-1 and TRF-2. TRF-2 protects chromosome ends, and TRF-1 regulates telomere length. ADP-ribosylation inhibits the ability of TRF-1 to bind to telomeric DNA. This poly-ADP-ribosylation of TRF-1 releases TRF-1 from the telomeres, thereby opening up the telomeric complex and allowing access to telomerase. Therefore, TANKs functions as positive regulators of telomere length, allowing elongation of the telomeres by telomerase.
Other roles for TANKs are suggested by the identity of proteins with which they interact—the insulin-responsive aminopeptidase, the Mcl1 proteins (which are members of the Bcl-2 family), the Epstein-Barr nuclear antigen-1, the nuclear and mitotic apparatus protein and the cytoplasmic and heterochromatic factor TAB182—and its various subcellular localizations (nuclear pores, Golgi apparatus and mitotic centrosomes).
Tankyrase-2 (TANK-2) differs from tankyrase-1 (TANK-1) in that it lacks an N-terminal HPS domain (comprised of homopolymeric repeats of His, Pro and Ser residues), found in TANK1. However, it probably has some overlapping functions with tankyrase-1, given that both proteins have similar sub-cellular localizations, associate with each other and bind many of the same proteins.
TANK-1 seems to be required for the polymerization of mitotic spindle-associated poly(ADP-ribose). The poly(ADP-ribosyl)ation activity of TANK-1 might be crucial for the accurate formation and maintenance of spindle bipolarity. Furthermore, PARP activity of TANK-1 has been shown to be required for normal telomere separation before anaphase. Interference with tankyrase PARP activity results in aberrant mitosis, which engenders a transient cell cycle arrest, probably due to spindle checkpoint activation, followed by cell death. Inhibition of tankyrases is therefore expected to have a cytotoxic effect on proliferating tumour cells.
As indicated above, the subcellular localization of several PARPs suggests a physiological role of poly(ADP-ribosyl)ation in the regulation of cell division.
PARP-1 and PARP-2 localize to centrosomes where they interact with kinetochore proteins. Ablation of the Parp-2 gene in mice causes significant DNA-damage-induced chromosome mis-segregation that is associated with kinetochore defects, which indicates that PARP-2 has a crucial guardian function in pericentric heterochromatin integrity. Furthermore PARP-1 associate with centrosomes linking the DNA-damage-surveillance network with the mitotic fidelity checkpoint.
The pivotal role of PARP in the repair of DNA strand breaks is well established, especially when caused directly by ionizing radiation or, indirectly after enzymatic repair of DNA lesions induced by methylating agents, topoisomerases I inhibitors and other chemotherapeutic agents as cisplatin and bleomycin. A variety of studies using “knockout” mice, trans-dominant inhibition models (over-expression of the DNA-binding domain), antisense and small molecular weight inhibitors have demonstrated the role of PARP in repair and cell survival after induction of DNA damage. The inhibition of PARP enzymatic activity should lead to an enhanced sensitivity of the tumour cells towards DNA damaging treatments.
PARP inhibitors have been reported to be effective in radiosensitizing (hypoxic) tumour cells and effective in preventing tumour cells from recovering from potentially lethal and sublethal damage of DNA after radiation therapy, presumably by their ability to prevent DNA strand break rejoining and by affecting several DNA damage signaling pathways.
U.S. Pat. No. 5,177,075 discusses several isoquinolines used for enhancing the lethal effects of ionizing radiation or chemotherapeutic agents on tumour cells. Weltin et al., (“Effect of 6(5-Phenanthridinone), an Inhibitor of Poly(ADP-ribose) Polymerase, on Cultured Tumour Cells”, Oncol. Res., 6:9, 399-403 (1994)), discusses the inhibition of PARP activity, reduced proliferation of tumour cells, and a marked synergistic effect when tumour cells are co-treated with an alkylating drug.
Reviews of the state of the art has been published by Li and Zhang in IDrugs 2001, 4(7): 804-812, by Ame et al in Bioassays 2004, 26: 882-883 and by Nguewa et al., in Progress in Biophysic & Molecular Biology 2005, 88: 143-172.
Loss of PARP-1 increases the formation of DNA lesions that are repaired by homologous recombination without directly regulating the process of homologous recombination itself. Familial breast cancer is commonly associated with inherited defects in one of the BRCA1 or BRCA2 alleles. BRCA1 and BRCA2 are important for homologous recombination. The remaining functional BRCA1 or BRCA2 allele can be lost in some cells, thereby contributing to tumourigenisis. Thus, the tumours that arise are BRCA1 or BRCA2 deficient (e.g. BRCA2−/−) whereas the somatic cells retain functional BRCA proteins (BRCA2+/−). Inhibition of PARP activity in a BRCA1- or BRCA2-defective background might result in the generation of DNA lesions normally repaired by sister chromatid exchange, causing chromatid aberrations and loss of viability. Only relatively low levels of PARP-1 inhibitors may be required to produce a therapeutic effect given the acute sensitivity of the BRCA-defective cells. This is another example of a case where inhibitors of a normally non-essential DNA repair protein can be used as a single agent to treat tumours.
According to a review by Horvath and Szabo (Drug News Perspect 20(3), April 2007, 171-181) most recent studies demonstrated that PARP inhibitors enhance cancer cell death primarily because they interfere with DNA repair on various levels. More recent studies have also demonstrated that PARP inhibitors inhibit angiogenesis, either by inhibiting growth factor expression; or by inhibiting growth factor-induced cellular proliferative responses. These findings might also have implications on the mode of PARP inhibitors' anticancer effects in vivo.
Also a study by Tentori et al, Eur. J. Cancer, 2007, doi: 10.1016/j.ejca2007.07010 (in press) shows that PARP inhibitors abrogate VEGF or placental growth factor-induced migration and prevent formation of tubule-like networks in cell-based systems, and impair angiogenesis in vivo. The study also demonstrates that growth factor-induced angiogenesis is deficient in PARP-1 knock-out mice. The results of the study provide evidence for targeting PARP for anti-angiogenesis, adding novel therapeutic implications to the use of PARP inhibitors in cancer treatment.
There continues to be a need for effective and potent anti-cancer therapy that produce minimal side effects. The present invention provides compounds, compositions for, and methods of, inhibiting PARP activity for treating cancer. Furthermore they are useful in enhancing the effectiveness of chemotherapy and radiotherapy where a primary effect of the treatment with the compound is that of triggering cell death under conditions of DNA damage.